1. Field of the Invention
The field of the present invention pertains to the field of power supplies and among other things to feedback for the regulation of power supplies.
2. Background of the Invention
Power supplies that convert an AC mains or a DC voltage to a DC voltage for use by integrated electronic devices, amongst other devices, are known. The power supplies are required to maintain the output voltage, current or power within a regulated range for efficient and safe operation of the electronic device. Switches that operate to maintain the output voltage, current, or power of the power supply within a regulated range are also known. Example of such switches include pulse width modulated switches. These switches utilize an oscillator and related circuitry to vary the pulse width or switching frequency of the switch, and therefore regulate the power, current or voltage that is supplied by the power supply. A switch that utilizes pulse width modulation control, utilizes feedback from the output of the power supply in order to maintain the output power, voltage or current within the regulated limits by varying its duty cycle.
A known power supply utilizing optocoupler feedback is depicted in FIG. 1. A bridge rectifier 5 rectifies an input AC mains voltage 10, which is smoothed into a substantially DC voltage 15 by power supply capacitor 20. The substantially DC voltage 15 is provided to primary winding 25 of coupled inductor 30 which is also known as a power transformer in power supply circuits. Coupled inductors 30 transfer energy according to the operation of a switch 35, that operates according to pulse width modulated control. When the switch 35 is closed it allows current to flow through primary winding 25. When switch 35 opens, current is induced in secondary winding 40. The current induced in the secondary winding 40 flows in the forward direction of diode 45 and is then filtered by secondary capacitor 50. The voltage at the secondary capacitor 50 is provided to the output of the power supply.
A feedback circuit 52 provides a signal indicative of the output voltage to the switch control circuitry 55 which then varies the duty cycle of the switch 35 with the magnitude of the signal indicative of the output voltage. The feedback circuit 52 includes a Zener diode 60 in series with a resistor 70. The resistor 70 is in series with a light emitting diode 75 of optocoupler 80. The optocoupler 80 also includes a phototransistor 85 that will allow current to flow when a current flows through light emitting diode 75. A bias winding 90, which is connected to a diode 92 and capacitor 94 supplies voltage so that a current can flow to the switch control circuitry 55 when phototransistor 85 is conducting.
The power supply of FIG. 1 or any other power supply that utilizes a coupled inductor can operate in either a continuous conduction mode or a discontinuous conduction mode. Referring to FIG. 2A, the core flux of the coupled inductor of a power supply in continuous conduction mode is depicted. During time period 100 current is flowing through the primary winding and the flux increases in the core of the coupled inductor to some level that is generally below the saturation level of the core. At the beginning of time 102, a switch such as switch 35 is opened and no further current is provided through the primary winding. During time 102 energy is delivered to one or more secondary windings that are coupled through the core with the primary winding. At the end of time 102, the flux in the core is greater than or equal to zero. The actual flux level that is stored at the end of time 102 is a function of the inductance of the core, the flux that was stored in the core during time 100 and the length of time 102. The combined times 100 and 102 are referred to as a cycle of operation of the power supply. Continuous conduction mode is generally utilized for high power consumption applications.
Referring to FIG. 2B, the core flux of the coupled inductor of a power supply operating in discontinuous conduction mode is depicted. During time 104 a current flows in the primary winding and flux in the core is increasing. At the beginning of time 106, a switch such as switch 35 is opened and no further current is provided through the primary winding. During time 106 energy is transferred to one or more secondary windings and the flux stored in the core is reduced. At the end of time 106 all the energy stored in the core has been transferred to the secondary winding and flux in the core is essentially zero. During time 108 no energy is transferred across the coupled inductor and the coupled inductor is idle. Discontinuous conduction mode generally occurs in low power consumption applications. Other power supply configurations can have an idle period of essentially zero flux in the transformer even in high power consumption applications.
A problem associated with the power supply of FIG. 1, regardless of whether it operates in continuous or discontinuous conduction mode, includes the addition of a bias winding 90, diode 92, capacitor 94 and optocoupler 80 which add to the cost and size of the power supply.
An additional problem associated with the power supply of FIG. 1 is that using an optocoupler 80 may result in unreliable isolation between the light emitting diode 75 and phototransistor 85 which are a relatively short distance apart and have a visual path between them. This arrangement of the light emitting diode 75 and phototransistor 85 may cause a short circuit between the light emitting diode 75 and phototransistor 85. If a short circuit occurs in the optocoupler then there is also a short circuit between the primary winding and secondary winding. Under these circumstances, the user of an electronic device which utilizes the power supply may be injured since the potentially lethal voltage at the primary side of the power supply is applied to the secondary side and potentially to the user. This issue is further problematic since it is difficult to test the isolation of an optocoupler once it is in the power supply. Furthermore, the optocoupler's isolation may fail without indication after the power supply leaves the factory. The quality of the isolation provided by the optocoupler is a function of its cost. Therefore, for low cost applications the use of an optocoupler can be problematic and will add to the cost of the electronic device.